1. Field of the Invention
The present invention relates to a cellular system using multi-hop relaying. More particularly, the present invention relates to an apparatus and method for supporting distributed spatial multiplexing and distributed spatial diversity.
2. Description of the Related Art
In order to provide a throughput higher than that provided by a 3rd Generation (3G) mobile communication system and to extend a service coverage area, there is a demand for development of a new 4th Generation (4G) mobile communication system. Thus, a competition on developments of a 4G standard is taking place in many research centers and businesses in technologically advanced countries.
The 4G mobile communication system which operates in a high frequency region experiences constraint in a throughput and a service coverage area due to a high path loss. To address such problems, researches based on a multi-hop signal transmission method are being conducted in recent years. According to a multi-hop technique, a path loss can be reduced when a Relay Station (RS) is used to relay data, and thus high speed data communication can be achieved. Furthermore, the service coverage area can be extended since a signal can be transmitted even when a Mobile Station (MS) is located far away from a Base Station (BS).
As such, in a multi-hop relay system, at least one RS is provided to relay data between a transmitter and a receiver. In a cellular system, the RS may be provided separately from the BS and the MS, or one MS may act as an RS with respect to another MS. Communications between two nodes in the multi-hop relay system may be achieved though a direct wireless link, i.e., a transmitter-RS link, an RS-RS link, an RS-receiver link and the like.
FIG. 1 illustrates a path for transmitting one frame in a conventional multi-hop relay system by using N−1 RSs.
Referring to FIG. 1, N resource units are used in the transmission of one frame. The resource units can be represented along the time axis or the frequency axis. For convenience of explanation, it will be assumed hereinafter that the resource units are represented using time slots. In a slot 1, a transmitter transmits a frame 1 to an RS1. In a slot N−1, an RS N−2 transmits the frame 1 received in its previous slot to an RS N−1. In a slot N, the RS N−1 transmits the frame 1 received in its previous slot to a receiver.
A multi-hop relay scheme may use either an Amplify & Forward method or a Decode & Forward method. In the Amplify & Forward method, an RS amplifies a Radio Frequency (RF) signal received from a transmitter, and then the RF signal is relay-transmitted to a receiver. An implementation of this method is simple, but noise enhancement is a problem because noise is also amplified when the RS amplifies the RF signal. On the other hand, in the Decode & Forward method, an RS demodulates and decodes a received signal, and then modulates and encodes the signal to be relayed. In comparison with the Amplify & Forward method, the Decode & Forward method has a demerit in that implementation is complex and error propagation may be frequent. However, the error propagation problem can be addressed when a link control protocol is properly designed, and thus the Decode & Forward method is advantageous over the Amplify & Forward method in terms of flexible technological scalability. A selection relaying protocol and an incremental relaying protocol are typical examples of a link control protocol combined with the Decode & Forward method.
Recently, as an advanced Decode & Forward method, a cooperation diversity technique has been introduced in which the concept of transmit antenna diversity is applied to the multi-hop relay scheme. In this technique, direct communication between a transmitter and a receiver is combined with multi-hop communication, and a plurality of signal transmission paths are formed between the transmitter and the receiver so that a diversity gain can be obtained together with a gain resulted from multi-hop transmission. This technique may be applied to a Distributed Spatial Diversity (DSD) system since a transmitter and an RS have multiple antennas which are spatially distributed and through which signals are transmitted. Therefore, a transmission diversity technique such as Space-Time Coding (STC) is needed to obtain a diversity gain.
FIG. 2 illustrates a path for transmitting one frame in a conventional DSD system.
Referring to FIG. 2, in a slot 1, a transmitter transmits a frame 1 to an RS 1. In a slot 2, the transmitter and the RS 1 transmit STC signals to an RS 2. In a slot N, the transmitter and RSs 1 to N−1 simultaneously transmit STC signals to a receiver. As a result, a diversity gain is obtained.
The multi-hop relay technique and the DSD technique, each of which use the aforementioned Amplify & Forward method or Decode & Forward method, are used under the premise that a transmission time and a reception time are different. In general, in a hardware implementation, an RS cannot simultaneously transmit and receive data using the same frequency band since transmission signal power is significantly greater than reception signal power. Therefore, an additional wireless resource has to be allocated for communication of a transmitter-RS link and an RS-RS link. This is a serious overhead when data is transmitted to the receiver. Thus, in the multi-hop relay and DSD techniques, data can be transmitted more reliably than when data is directly transmitted between the transmitter and the receiver, whereas throughput deterioration occurs due to the aforementioned overhead. For example, when one RS is used, if data is transmitted with a constant throughput via a transmitter-RS link and an RS-receiver link, the throughput may be 50% of the case when data is directly transmitted between the transmitter and the receiver.
The aforementioned problem of the throughput deterioration may be reduced when using an adaptive modulation technique. However, implementation thereof is complex because channel information has to be fed back from one node to another and because of complex signaling. According to the distributed spatial multiplexing, the throughput deterioration problem can be solved by using a multiplexing technique instead of using a diversity scheme for a geographically distributed antenna. That is, when it is allowed for a transmitter and one or more RSs to be able to simultaneously transmit different data, and when a receiver having multiple antennas can detect the different data by performing Multi-Input Multi-Output (MIMO) detection, then a high throughput can be achieved.
FIG. 3 illustrates a path for transmitting N frames in a conventional distributed spatial multiplexing system.
Referring to FIG. 3, in a slot 1, a transmitter transmits a frame 1 to an RS 1. Then, the RS 1 detects the frame 1 and stores it in its buffer. In a slot 2, the transmitter transmits a frame 2 to an RS 2, and the RS 2 detects the frame 2 and stores it in its buffer. Likewise, such process is performed by N−1 times so that a total of N−1 RSs can store different frames. Finally, in a slot N, the transmitter transmits a frame N, and the N−1 RSs simultaneously transmit the different frames stored in their buffers. As a result, N frames are transmitted during N slots, and thus it is possible to provide the same throughput as direct transmission.
Consequently, in the DSD technique, spatial diversity can be obtained by transmitting the same frame through several relay paths using the STC. However, disadvantageously, the DSD technique experiences throughput deterioration as N times as that of the direct transmission. On the other hand, in the distributed spatial multiplexing technique, the same throughput as the direct transmission can be obtained when different frames are transmitted through different relay paths and are detected by performing MIMO detection in a receiver. However, the distributed spatial multiplexing technique has a demerit in that diversity effect cannot be expected.
Accordingly, there is a need for a method capable of supporting advantageous functions of the two techniques. In other words, a new scheme is demanded whereby a diversity gain of the DSD technique and a high throughput of the distributed spatial multiplexing technique are both achieved.